Impedance transformer



P 1933. c. w. HANSELL ,926,807

IMPEDANCE TRANSFORMER Filed April 14, 1928 INVENTOR CLARENCE w HANSELL ORNEY Patented Sept. 12, 1933 UNITED STATES PATENT OFFICE IMPEDANCE TRANSFORMER Clarence W. Hansell,

Rocky Point, N. Y.,

assignor to Radio Corporation of America, a corporation of Delaware Application April 14, 1928. Serial No. 269,939

Claims.

This invention relates to impedance transformers, and more particularlyto a novel transformer consisting of a line the inductance and capacitance characteristics of which are varied to V p 5' obtain the desired impedance transformation.

At high frequencies it becomes exceedingly difllcult to construct efficient transformers, and accordingly the tendency is towards various forms of direct coupling which, by a suitable combination of reactances, serve to produce the desired impedance change. These methods are accompanied by the disadvantage that the degree bf coupling and transformation varies with the frequency, a difliculty which is experienced to a considerable degree even in the range of lower frequencies Where the use of transformers is practicable, and which is still more marked with directly coupled reactance arrangements;

It is a primary object of my invention to pro- J vide an impedance transformer which is independent of frequency, or otherwise stated, which "experiences no resonance phenomena. This I propose to do by coupling the circuits through anatural or artificial'line the inductance and .155 capacitance characteristics of which are such that the impedance at oneend equals one of the desired impedances, and the impedance at the other end equals the other desired impedance, while the impedances at points intermediate the jpf'ends vary exponentially between the desired end iinpedances.

'Ihe underlying idea may be explained, by its analogy with the now well known exponential horn, which is no more than an air impedance changing device. At the diaphragm or small end of the horn there is a. high air impedance, while at the mouth or large end'of the horn there is a low air impedance, and the horn serves to change from one to the other. By varying the cross section of the horn exponentially along its length it is found that resonance phenomena within the horn may be effectually prevented. In a similar'way, I propose to couple two electrical circuits of differing electrical impedances with an impedance changing device which transforms the impedance gradually and progressively in amounts which vary in exponential or geometric progression, so that the impedance transformer is aperiodic.

'Ihe invention is described more in detail in connection with the accompanying drawing, in

which Figure 1 shows an artificial line built up of a number of sections each having series inductance 5 and shunt capacitance;

Figure 2 shows a similar arrangement having "series capacitance and shunt inductance;

Figure 3 indicates a line and antenna coupled together by an aperiodic transformer employing no lumped reactances;

Figure 4 is a modification; and

Figure 5- shows interstage coupling for exceedingly high frequencies.

Referring to Figure 1 there is an artifical line made up of a number of sections comprising series inductances 2 and shunt condensers 4. Now in accordance with the present invention if a total impedance change from R to r is required, and if X sections are to be employed, the im pedance ratio for each section, N, is made equal to the total impedance ratio, raised to an exponent which is the reciprocal of the number of sections, X. In this way, as can be seen from the notation below Fig. 2, the impedance is varied exponentially along the artificial line, so that after'five sections, as shown in Figure 1 as a simple illustration, the impedance is NR, and since this is to be equal to r, it is obvious that N equals the 5th root of the ratio The arrangement shown in Figure -1 may be explained more in detail as follows. 'It is well known that the surge impedance of a line having series inductance and shunt capacity is approximately Z /& C

1 =1- M/LC It should be noticed that if the impedance along the me is varied by increasing L and decreasing C each by the desired factor the impedance of the line will be increased by that factor,

inasmuch as r F h,

and at the same time the cut off frequency will remain constant, inasmuch as the product LC Will remain constant.

The legend L appearing in the equations above, represents the inductance per section or per unit of length of an artificial line. With reference to Fig. 1 for example, L is the portion of the inductance per section of the line which may be assigned to each of the capacities. Thus,

posed conductors whose areas vary substantially continuously along the length of the device, the impedance of said device varying exponentially along its length whereby its terminals present impedances having values which are equal to the impedances to be faced.

3. In combination, a pair of radio frequency electrical circuits having different impedances, an impedance matching device whose terminals are adapted to connect together said pair of circuits, each terminal of said device having an impedance whose value corresponds to the impedance of the electrical circuit which it faces, said device comprising a plurality of superposed conductors the impedance of which as a whole tapers continuously along the length of the device.

4. A radio frequency impedance matching device whose terminals are adapted to face different impedances comprising a plurality of superposed conductors, at least one of which varies smoothly and continuously in mass along its length, the impedance of said device tapering substantially continuously along its length whereby its tenninals present impedances having values which are equal to the impedances to be faced.

5. In combination, a radio transmitter having a predetermined impedance, a radiating antenna having a different impedance, and a radio frequency impedance matching device therebetween whose terminals are adapted to face said two different impedances comprising a plurality of conductors, at least one of which varies in mass substantially continuously along its length, the impedance of said device tapering substantially continuously along its length whereby its terminals present impedances having values which are equal to the respective impedances to be faced.

CLARENCE W. HANSELL. 

